The present invention relates to an printing apparatus capable of handling large size sheets. More specifically the invention is related to a colour proofing apparatus having a dual-side mode.
Printed products contain not only printed text but many photographic illustrations are included in all sorts of printed products such as leaflets, magazines etc . . .
Nowadays text and illustrations are usually provided in electronic form in order to electronically compose the publication wherein the photographs are used.
The photographs provided in an electronic format are usually provided as a continuous tone image. A continuous tone image is an image containing multiple grey (colour) levels with no perceptible quantization to them.
In order to reproduce the continuous tone images by a printing process having only a limited number of grey levels, a halftoning process is used.
Digital halftoning refers to the process whereby a digital (continuous tone) image is rendered with a computer-controlled graphics output device that is capable of generating only a limited number of grey (colour) levels. The input image is electronically screened to obtain a halftone image for each colour.
In order to inspect the correctness of the digital halftoning process on the final product printed on a printing press, the printed result is simulated using a colour proofing apparatus.
A modern colour proofing apparatus is capable of not only providing a simulated print of the photographs but of the whole printed product. Several large size colour proofers use an inkjet printing process to render the halftone images on a large sheet of paper or other receiving media. Sheet sizes used may vary but a used format may be typical 109.22 mm (43 inch)xc3x9776.2 mm (30 inch). The medium is usually of a paper type, typically 100-130 g/mm2, but also other media type can be used e.g. polyester supports which can be coated with appropriate ink receiving coatings etc . . .
These large sheets can be fed from a magazine but are frequently dispensed from a web coupled to the machine enabling the use of different lengths of receiving material which is cut at appropriate length. Depending on the form of the receiving medium, it can hereinafter be named receiving web or receiving sheet and is given the same number in the drawings of the application.
Recently, in order to completely imitate the final product, colour proofers having dual-side capabilities have emerged. With these apparatuses it is possible to imitate the dual-side printed output product and to control the imposition scheme, i.e. order and orientation of different pages or a large sheet which e.g. is folded and cut to render a small booklet or folder with numbered pages. One such apparatus is the Spinjet(copyright) sold by Techsage(copyright).
A new problem emerging is the lack of registering of the front- and backside images on the proofing product giving unsatisfactory results. In some colour proofers one can manually turn the receiving sheet and feed it again into the printer by hand using e.g. a registration edge as an aid. In other printers the medium is turned over automatically. Hitherto it has been a problem providing a good registering of the output image on the large size output media in use. Especially registering correctly the images on the front and back side of large output media used in the colour and imposition proofing apparatus has been a problem. The curl remaining in the paper further causes problems in feeding the paper correctly.
Until now non easy an reliable solution has been provided to this problem.
It is an object of the present invention to provide a apparatus having a mechanism capable of reliable and correctly registering an output image on a receiver.
It is a further object of the invention to provide a front to back registering system for a dual-side printing apparatus.
It is another object of the invention to provide an improved reversing mechanism for large size receiver media in a dual-side printing apparatus.
The above mentioned objects are realised by a apparatus having the specific features defined in claim 1. Specific features for preferred embodiments of the invention are set out in the dependent claims.
A printing method according to the invention is defined in claim 9.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.